<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Title</title>
  <style>
    html, body {
      height: 100%;
    }
    div {
      border: 1px solid #ccc;
    }
  </style>
</head>
<body>
<div style="display: flex;height: 100%">
  <div style="width: 100px"></div>
  <div style="width: 600px"></div>
</div>

</body>
</html>
